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Doctors have long known that high levels of stress may make people more vulnerable to illness, but now researchers report how psychological stress can wreak havoc on your health.

It turns out that cortisol — a stress hormone released when we feel anxious — plays a major role. Cortisol temporarily suppresses the immune system, reducing the body’s natural inflammatory response to viruses and bacteria. Chronic stress, however, throws the inflammatory response out of whack; the immune system becomes less sensitive to cortisol when stress becomes constant. So, rather than dampening inflammation, chronically high levels of cortisol have the opposite effect.

That’s why stressed people are more susceptible to the common cold, the researchers say. Cold symptoms like coughing and sneezing and are not caused directly by the cold virus, but as a byproduct of the body’s inflammatory response to the bug.

“Stressed people’s immune cells become less sensitive to cortisol,” lead author Dr. Sheldon Cohen, a professor of psychology at Carnegie Mellon University, in Pittsburgh, told CNN. “They’re unable to regulate the inflammatory response, and therefore, when they’re exposed to a virus, they’re more likely to develop a cold.”

The researchers conducted two experiments in the study. In the first, they interviewed 276 healthy adults on what had stressed them out — work, relationships, money — over the previous year. The participants were then given nasal drops containing a common cold virus to make them sick, and were quarantined for five days. About 39% of the participants developed a cold, and those who reported being stressed out were twice as likely as the others to become ill.

Participants’ overall levels of cortisol had no impact on their chances of getting sick, the researchers found. Rather, in stressed people, the body’s normal immune response to cortisol was disrupted.

In the second part of the study, the researchers analyzed the ability of 79 participants to regulate their inflammatory response, and then exposed them to a cold virus. Then they tracked the participants’ production of pro-inflammatory cytokines, chemical messengers that cause inflammation. Those who started out with poor inflammatory response regulation produced more of the cytokines when they “caught” the virus.

“The immune system’s ability to regulate inflammation predicts who will develop a cold, but more importantly it provides an explanation of how stress can promote disease,” Cohen said in a statement. “When under stress, cells of the immune system are unable to respond to hormonal control, and consequently, produce levels of inflammation that promote disease.”

By highlighting how stress makes the body vulnerable, the researchers hope their findings can help lead to better ways to protect health in the chronically stressed. “Because inflammation plays a role in many diseases such as cardiovascular, asthma and autoimmune disorders, this model suggests why stress impacts them as well,” Cohen said. “Knowing this is important for identifying which diseases may be influenced by stress and for preventing disease in chronically stressed people.”

The study was published in the journal Proceedings of National Academy of Sciences.